1. Technical Field
The present invention relates to a piezoelectric vibrator and an ultrasonic motor having the piezoelectric vibrator, more specifically to a piezoelectric vibrator and an ultrasonic motor having the piezoelectric vibrator that have simple compositions and small volumes and can improve vibration performance.
2. Description of the Related Art
Recently, the ultrasonic motor has received attention as a motor that does not require wound coils, to be suitable, for example, for decreasing the size of a device. The ultrasonic motor is widely used, as it entails low power consumption, has a light weight, provides linear motion directly without gears, allows the control of speed and position electrically, and allows movement in either the forward or reverse direction.
FIG. 1 is a plan view of a piezoelectric vibrator 10 used in a conventional ultrasonic motor.
A conventional piezoelectric vibrator 10 comprises a rectangular piezoelectric element 13 made of piezoelectric ceramic, etc., and a protrusion part 11 formed on a side of the piezoelectric element 13. The protrusion part 11 applies pressure on the object of vibration (not shown), where the protrusion part 11 moves the object of vibration due to the vibration of the piezoelectric element 13. There are four polarization regions, i.e. a first polarization region 13a, second polarization region 13b, third polarization region 13c, and fourth polarization region 13d, formed on the piezoelectric element 13, where all polarization regions 13a, 13b, 13c, 13d have the same polarization direction in the direction of thickness. The four polarization regions 13a, 13b, 13c, 13d have the same size and are arranged in two rows. On each of the four polarization regions 13a, 13b, 13c, 13d is formed an electrode.
The first and fourth polarization regions 13a, 13d have the same polarization direction, while the second and third polarization regions 13b, 13c have a polarization direction opposite to that of the first polarization region 13a. Also, the first and fourth polarization regions 13a, 13d and the second and third polarization regions 13b, 13c are connected respectively by a lead wire 17.
The piezoelectric element 13 vibrates in longitudinal and bending directions when an electric current is supplied to the first and fourth polarization regions 13a, 13d. Similarly; when an electric current is supplied to the second and third polarization regions, 13b, 13c, the piezoelectric element 13 vibrates in longitudinal and bending directions however, this time the direction of bending vibration is opposite to previous case.
Since the conventional piezoelectric vibrator 10 has two polarization directions on one piezoelectric element 13, as described above, two polarization processes are required. This entails the problems of increased manufacturing time and cost of the piezoelectric element. In particular, if the two polarization processes are performed separately on one piezoelectric element 13, depolarization may occur on the portions where the polarization is performed first, to consequently lower the performance of the piezoelectric element 13.
Further, in the conventional piezoelectric vibrator 10, only one pair of polarization regions 13a, 13d positioned diagonally are excited, while the other pair 13b, 13c are not, which lowers the vibration performance of the piezoelectric vibrator 10. This means that a higher voltage must be supplied to improve the vibration performance of the conventional piezoelectric vibrator 10. Moreover, to improve vibration performance, the conventional piezoelectric vibrator 10 was used with the piezoelectric elements 13 stacked in multiple layers, which incurs the problem of increased volume of the piezoelectric element.
Also, polarization involves supplying a high DC voltage to a piezoelectric element 13 to arrange the dipoles within the piezoelectric element 13 into a desired orientation, and during the polarization process, large amounts of stress are concentrated on the boundaries of the electrodes positioned in-between the stacked piezoelectric elements 13. Such stress becomes a major cause of cracks later during the operation of the piezoelectric vibrator 10, and deteriorates the properties of the piezoelectric element 13.